Many vehicles, in particular automobiles, are equipped with occupant detection devices so occupant protection devices such as airbags can be enabled or disabled based on who or what is occupying a seat in the vehicle. For example, if a seat is empty, or occupied by a small child, it may be desirable to reduce the deployment force or disable an airbag for that seat to prevent airbag deployment if the vehicle is involved in a collision. One way to estimate the size of a seat occupant is to equip the seat with a device that estimates occupant weight.
It has been observed that changes in seat orientation angle relative to the vehicle and/or the direction of the force of gravity, and changes in vehicle orientation angle relative to the direction of the force of gravity, may change the apparent weight of an occupant indicated by the device estimating occupant weight. As used herein, seat orientation angle means a difference between the forward/backward tilting (pitch angle) of the seat and/or left/right tilting (roll angle) of the seat, relative to some reference position and the direction of the force of gravity. Variations in seat/vehicle orientation angle change the occupant's force vector due to gravity with respect to the weight sensor. Also, the occupant weight can be ‘off-loaded’ from the weight sensor to other areas of the seat such as the edges of the seat, or the back-rest if the seat orientation angle changes. In addition, up/down motion (heave) induced by vehicle movement as the vehicle travels may also change the apparent weight of an occupant. According to some observations, the effects of seat orientation angle can cause a 10% error of full scale in weight estimation. The effects of heave may add to the problem. In one situation, the effects of seat orientation angle and heave can make distinguishing 5th percentile adults (small adults) from children difficult.